1. Field of the Invention
The present invention relates to a coupling device for rotationally coupling a drive shaft of a drive device and a driven shaft of an operation device on a common axis.
2. Discussion of the Related Art
For example, in a vehicle brake system, it is carried out to supply wheel cylinders with a hydraulic pressure which is generated in a master cylinder as a result that the driver steps a brake pedal, to apply a brake force to wheels by pressing brake pads against brake discs and at the same time, to apply a brake force to the wheels by generating a hydraulic pressure from a pump driven by an electric motor and by supplying the pump-generated hydraulic pressure to the wheel cylinders.
JP2004-68836 A (hereafter referred to as Patent Document 1) describes a coupling device, wherein a coupling recess 91 is formed on an end portion of a motor shaft 90 of an electric motor in a vehicle brake system, wherein a coupling protrusion 81 which is engaged with the coupling recess 91 is formed on an end portion of a pump shaft 80 of a pump, and wherein opposite inner wall surfaces of the coupling recess 91 press radially outer edge corners of the coupling protrusion 81 to transmit a rotation power from the motor shaft 90 to the pump shaft 80.
In the coupling device described in Patent Document 1, since the rotation power is transmitted as a result that the opposite inner wall surfaces of the coupling recess 91 press the acute radially outer edge corners of the coupling protrusion 81, a surface pressure (i.e., contact strength) at each of the contact portions between the coupling recess 91 and the coupling protrusion 81 becomes high, and the opposite inner wall surfaces of the coupling recess 91 and the radially outer edge corners of the coupling protrusion 81 wear within a short period of time in use. Thus, it occurs that the rotation power is transmitted with the worn radially outer edge corners of the coupling protrusion 81 fitted in depressed grooves which are formed on the coupling recess 91 due to the wear, and this gives rise to an apprehension that smooth and gentle transmission of the rotation power from the electric motor to the pump becomes impossible within a short period of service life. In particular, the aforementioned wear is accelerated by the relative movement in radial direction which is caused between the shaft 90 of the electric motor and the shaft 80 of the pump due to a slight axial misalignment therebetween.
Further, in recent years, in vehicle brake systems, the use of an antilock brake system, a traction control system, a yaw control system or the like has been increasing, or a so-called “by-wire brake system” has been developed which generates a hydraulic pressure from a pump driven by an electric motor in proportion to a hydraulic pressure generated as a result of the driver stepping a brake pedal and which applies the pump-generated hydraulic pressure to wheel cylinders to apply an ordinary brake force to wheels. This results in remarkably increasing the frequency and time period in use of the pump driven by the electric motor in the vehicle brake system. Further, in vehicle brake systems, the electric motor and the pump are required to be reduced in dimension and weight, wherein the motor shaft and the pump shaft are required to be decreased in diameter. With decreases in diameter of the shafts, a surface pressure is increased at the contact portion between the coupling recess and the coupling protrusion of the coupling device, which results in further shortening the service life of the coupling device. Under these circumstances, it is strongly desired to extend the service life of the coupling device.